The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in the high-tech electronic industries. Studies on rechargeable lithium batteries are actively being pursued in accordance with an increased need for batteries having high energy density for use as power sources in these portable electronic instruments. Even though graphite is suggested for the negative active material for such batteries as it has a theoretical capacity of 372 mAh/g, a material with a higher capacity than graphite is desired.
Silicon or silicone compounds have been proposed as a substitute for graphite. The silicon or silicone compounds can be alloyed with lithium and have a higher electric capacity than graphite.
Recently, the following have been proposed as substitutes for conventional graphite material: (a) a simple mixture of graphite and a silicone compound powder; (b) a material in which a pulverized silicone compound is chemically fixed on the surface of graphite by a silane coupling agent; and (c) a material in which an element such as Si is bound with or coated on a graphite-based carbonaceous material.
However, regarding (a) a simple mixture of graphite and a silicone compound powder, the graphite is not completely contacted with the silicone compound so that the silicone compound is released from the graphite when the graphite is expanded or contracted upon repeating the charge and discharge cycles. Therefore, as the silicone compound has low electro-conductivity, the silicone compound is insufficiently utilized for a negative active material and the cycle characteristics of the rechargeable lithium battery deteriorate.
Regarding (b) a material in which a pulverized silicone compound is chemically fixed on the surface of graphite by a silane coupling agent, although the resulting material works as a negative active material, at the early charge and discharge cycles, problems arise in that the silicone compound expands when it is alloyed with the lithium upon repeating the charge and discharge cycles. Therefore, the linkage of the silane coupling agent is broken, releasing the silicone compound from the graphite so that the silicone compound is insufficiently utilized as a negative active material. As a result, the cycle characteristics of the rechargeable lithium battery deteriorate. Further, the silane coupling agent may not be uniformly treated upon preparing the negative electrode material so that it is difficult to provide a negative electrode material having a constant quality.
Regarding (c) a material in which an element such as Si is bound with or coated on the graphite-based carbonaceous material, such a material has similar problems as those of (b) a material in which the pulverized silicone compound is chemically fixed on the surface of graphite by a silane coupling agent. That is, upon progressing through charge and discharge cycles, the linkage of the amorphous carbonaceous material can be broken by the expansion of the material alloyed with the lithium. The material is thereby released from the graphite carbonaceous material and is insufficiently utilized as a negative active material. As a result, the cycle characteristics deteriorate.